U.S. patent application number 13/239091 was filed with the patent office on 2012-04-19 for self cleaning large scale method and furnace system for selenization of thin film photovoltaic materials.
This patent application is currently assigned to Stion Corporation. Invention is credited to Robert D. Wieting.
Application Number | 20120094432 13/239091 |
Document ID | / |
Family ID | 44560387 |
Filed Date | 2012-04-19 |
United States Patent
Application |
20120094432 |
Kind Code |
A1 |
Wieting; Robert D. |
April 19, 2012 |
SELF CLEANING LARGE SCALE METHOD AND FURNACE SYSTEM FOR
SELENIZATION OF THIN FILM PHOTOVOLTAIC MATERIALS
Abstract
A method for fabricating a copper indium diselenide
semiconductor film using a self cleaning furnace is provided. The
method includes transferring a plurality of substrates having a
copper and indium composite structure into a furnace comprising a
processing region and at least one end cap region disengageably
coupled to the processing region. The method also includes
introducing a gaseous species including a hydrogen species and a
selenium species and a carrier gas into the furnace and
transferring thermal energy into the furnace to increase a
temperature from a first temperature to a second temperature to
initiate formation of a copper indium diselenide film on each of
the substrates. The method further includes decomposing residual
selenide species from an inner region of the process region of the
furnace. The method further includes depositing elemental selenium
species within a vicinity of the end cap region operable at a third
temperature.
Inventors: |
Wieting; Robert D.; (San
Jose, CA) |
Assignee: |
Stion Corporation
San Jose
CA
|
Family ID: |
44560387 |
Appl. No.: |
13/239091 |
Filed: |
September 21, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12568656 |
Sep 28, 2009 |
8053274 |
|
|
13239091 |
|
|
|
|
61101651 |
Sep 30, 2008 |
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Current U.S.
Class: |
438/102 ;
118/724; 257/E21.463 |
Current CPC
Class: |
H01L 21/02568 20130101;
H01L 21/67109 20130101; Y10S 438/905 20130101; C23C 14/5866
20130101; H01L 31/0322 20130101; Y02E 10/541 20130101; C23C 14/185
20130101; H01L 21/67028 20130101; C23C 14/564 20130101; H01L
21/02614 20130101 |
Class at
Publication: |
438/102 ;
118/724; 257/E21.463 |
International
Class: |
H01L 21/365 20060101
H01L021/365; C23C 16/44 20060101 C23C016/44; C23C 16/22 20060101
C23C016/22 |
Claims
1. A method comprising: transferring a plurality of substrates into
a furnace, the furnace comprising a processing region and at least
one end cap region disengageably coupled to the processing region,
wherein each of the plurality of substrates includes a copper and
indium composite structure; introducing a gaseous species including
a hydrogen species and a selenide species into the furnace;
transferring thermal energy into the furnace to increase a
temperature of the furnace from a first temperature to a second
temperature; initiating formation of a copper indium diselenide
film from the copper and indium composite structure; decomposing
residual selenide species from an inner region of the processing
region; changing the temperature of the furnace from the second
temperature to a third temperature; depositing elemental selenium
species within a vicinity of the at least one end cap region;
changing the temperature of the furnace from the third temperature
to the first temperature; and removing the remaining gaseous
species from the chamber.
2. A method comprising: transferring a plurality of substrates into
a furnace, wherein each of the plurality of substrates includes a
copper and indium composite structure; introducing a gaseous
species including a hydrogen species and a selenide species into
the furnace; transferring thermal energy into the furnace to
increase a temperature of the furnace from a first temperature to a
second temperature; initiating formation of a copper indium
diselenide film from the copper and indium composite structure;
decomposing residual selenide species from an inner region of the
processing region; changing the temperature of the furnace from the
second temperature to a third temperature; depositing elemental
selenium species within a vicinity of a end-cap region of the
furnace; changing the temperature of the furnace from the third
temperature to the first temperature; and removing the remaining
gaseous species from the chamber.
3. A system comprising: a process chamber having an inner process
region; an end cap coupled to the process chamber and including an
end cap region; and one or more heating elements coupled to the
process chamber; wherein the system is configured to: hold a
plurality of substrates in the process chamber, each of the
plurality of substrates including a copper and indium composite
structure; introduce a gaseous species including a hydrogen species
and a selenide species into the process chamber; heat the process
chamber from a first temperature to a second temperature using the
one or more heating elements; initiate formation of a copper indium
diselenide film from the copper and indium composite structure;
cause decomposition of residual selenide species from the inner
process region; change the temperature of the process chamber from
the second temperature to a third temperature; deposit elemental
selenium species within a vicinity of the end cap region; cause
formation of copper indium diselenide film over each of the
plurality of substrates; and remove remaining gaseous species from
the process chamber not used in the formation of the copper indium
diselenide film.
4. The system of claim 3 further comprising a gas injection pipe
configured to introduce the gaseous species into the process
chamber.
5. The system of claim 3 wherein the end cap is disengageably
coupled to the process chamber.
6. The system of claim 3 wherein the remaining gaseous species are
removed from the process chamber using a turbomolecular pump.
7. The system of claim 3 wherein the end cap comprises metal.
8. The system of claim 3 wherein the process chamber comprises a
quartz tube.
9. The system of claim 3 wherein the process chamber is
characterized by a first specific heat value and the end cap is
characterized by a second specific heat value, wherein the second
specific value is lower than the first specific heat value.
10. The system of claim 3 wherein the second temperature ranges
between 350.degree. C. and 450.degree. C.
11. The system of claim 3 wherein the third temperature ranges
between 500.degree. C. and 525.degree. C.
12. The system of claim 3 wherein the end cap is characterized by a
first thermal conductivity and the process chamber is characterized
by a second thermal conductivity and wherein the first thermal
conductivity is lower than the second thermal conductivity.
13. The system of claim 12 further configured to cause creation of
convection current within the inner process region.
14. The system of claim 3 wherein the one or more heating elements
are individually controllable.
15. The system of claim 3 further configured to: decompose residual
selenide species from the inner region of the process chamber; and
remove a first portion of selenium from the copper indium
diselenide film and replace it with a second portion of sulfur.
16. The system of claim 15 wherein the first portion and the second
portion are about 5% each.
17. The system of claim 3 further comprising a baffle disposed
between the inner region of the process chamber and the end
cap.
18. The system of claim 3 wherein the plurality of substrates are
held in a vertical orientation with respect to gravity.
19. The system of claim 3 wherein the end cap is configured to be
separable from the process chamber to allow mechanical cleaning of
the end cap.
20. The system of claim 3 wherein the process chamber is
characterized by a pressure of about 650 Torr.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority to
U.S. application Ser. No. 12/568,656 filed on Sep. 28, 2009, which
in turn claims priority to U.S. Provisional Patent Application No.
61/101,651, filed Sep. 30, 2008, entitled "SELF CLEANING LARGE
SCALE METHOD AND FURNACE SYSTEM FOR SELENIZATION OF THIN FILM
PHOTOVOLTAIC MATERIALS" by inventor Robert D. Wieting, the contents
of both these applications are incorporated by reference herein in
their entirety for all purposes.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to photovoltaic
techniques. More particularly, the present invention provides a
method and structure for a thin film photovoltaic device using a
copper indium diselenide species (CIS), copper indium gallium
diselenide species (CIGS), and/or others. The invention can be
applied to photovoltaic modules, flexible sheets, building or
window glass, automotive and others.
[0003] In the process of manufacturing CIS and/or CIGS types of
thin films, there are various manufacturing challenges, such as
maintaining structure integrity of substrate materials, ensuring
uniformity and granularity of the thin film material, etc. While
conventional techniques in the past have addressed some of these
issues, they are often inadequate in various situations. Among
others, it is often difficult to clean systems (e.g., processing
chambers) that is used to manufacture CIS and/or CIGS films.
Therefore, it is desirable to have improved systems and method for
manufacturing thin film photovoltaic devices.
BRIEF SUMMARY OF THE INVENTION
[0004] The present invention relates generally to photovoltaic
techniques. More particularly, the present invention provides a
method and structure for a thin film photovoltaic device using a
copper indium diselenide species (CIS), copper indium gallium
diselenide species (CIGS), and/or others. The invention can be
applied to photovoltaic modules, flexible sheets, building or
window glass, automotive and others.
[0005] According to an embodiment, the present invention provide a
method for fabricating a copper indium diselenide semiconductor
film using a self cleaning furnace. The method includes
transferring a plurality of substrates into a furnace, the furnace
comprising a processing region and at least one end cap region
disengageably coupled to the processing region, each of the
plurality of substrates provided in a vertical orientation with
respect to a direction of gravity, the plurality of substrates
being defined by a number N, where N is greater than 5, each of the
substrates having a copper and indium composite structure. The
method also includes introducing a gaseous species including a
hydrogen species and a selenide species and a carrier gas into the
furnace and transferring thermal energy into the furnace to
increase a temperature from a first temperature to a second
temperature, the second temperature ranging from about 350 Degrees
Celsius to about 450 Degrees Celsius to at least initiate formation
of a copper indium diselenide film from the copper and indium
composite structure on each of the substrates. The method further
includes decomposing residual selenide species from an inner region
of the process region of the furnace. The method further includes
depositing elemental selenide species within a vicinity of the end
cap region operable at a third temperature. Also, the method
includes maintaining the inner region substantially free from
elemental selenide species by at least the decomposition of
residual selenide species from the inner region of the process
region.
[0006] In a specific embodiment, the hydrogen selenide gas is used
as work gas in the furnace. As the temperature is around
400.degree. C. or greater, hydrogen selenide gas is thermally
activated to be pyrolyzed, forming elemental selenium (Se) or
selenium clusters (Se.sub.2 or Se.sub.3). The end cap region
includes a lid with temperature controlled by running active water
for cooling and using lamps for heating. The temperature of the lid
is monitored to keep cool to serve as a "cyropump" so that the
selenium species (elemental selenium and selenium cluster) can
deposit over the lid and the reaction between selenium and other
film material (such as Indium) is suppressed. After finishing one
or more process cycles, the lid of the end cap region can be
further cleaned with a cloth, for example linseed cloth, or similar
material to remove the deposited selenium residues and
particles.
[0007] It is to be appreciated that the present invention provides
numerous benefits over conventional techniques. Among other things,
the systems and processes of the present invention are compatible
with conventional systems, which allow cost effective
implementation. In various embodiments, residue gases are
aggregated into a specific region of processing chamber to allow
easy cleaning. There are other benefits as well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a simplified diagram of a transparent substrate
with an overlying electrode layer according to an embodiment of the
present invention;
[0009] FIG. 2 is a simplified diagram of a composite structure
including a copper and indium film according to an embodiment of
the present invention;
[0010] FIG. 2A is a simplified diagram of a composite structure
including a copper indium composite/alloy according to an
embodiment of the present invention;
[0011] FIG. 3 is a simplified diagram of a furnace according to an
embodiment of the present invention;
[0012] FIG. 4 is a simplified diagram of a process for forming a
copper indium diselenide layer according to an embodiment of the
present invention;
[0013] FIGS. 5 and 5A are simplified diagrams of a temperature
profile of the furnace according to an embodiment of the present
invention;
[0014] FIG. 6A is a simplified diagram of a thin film copper indium
diselenide device according to an embodiment of the present
invention;
[0015] FIG. 6B is a simplified diagram of a thin film copper indium
diselenide device according to another embodiment of the present
invention;
[0016] FIG. 7 is a simplified diagram of a self cleaning furnace
before the decomposition of the selenide species according to an
embodiment of the present invention;
[0017] FIG. 8A is a simplified diagram of a self cleaning furnace
before the decomposition of the selenide species according to an
embodiment of the present invention;
[0018] FIG. 8B is a simplified diagram of a self cleaning furnace
during the decomposition of the selenide species and deposition of
elemental selenium at the end cap according to an embodiment of the
present invention;
[0019] FIG. 8C is a simplified diagram of a self cleaning furnace
after the decomposition of the selenide species and deposition of
elemental selenium at the end cap according to an embodiment of the
present invention; and
[0020] FIG. 9 is a simplified diagram of a self cleaning furnace
after the decomposition of the selenide species and deposition of
elemental selenium at the end cap according to an embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention relates generally to photovoltaic
techniques. More particularly, the present invention provides a
method and structure for a thin film photovoltaic device using a
copper indium diselenide species (CIS), copper indium gallium
diselenide species (CIGS), and/or others. The invention can be
applied to photovoltaic modules, flexible sheets, building or
window glass, automotive, and others.
[0022] FIG. 1 is a simplified diagram of a transparent substrate
with an overlying electrode layer according to an embodiment of the
present invention. This diagram is merely an example, which should
not limit the scope of the claims herein. As shown, structure 100
includes a transparent substrate 104. In an embodiment, substrate
104 can be a glass substrate, for example, a soda lime glass.
However, other types of substrates can also be used. Examples of
substrates include borosilicate glass, acrylic glass, sugar glass,
specialty Corning.TM. glass, and others. As shown, a contact layer
comprising a metal electrode layer 102 is deposited upon substrate
104. According to an embodiment, the metal electrode layer 102
comprises metal material that is characterized by a predetermined
conductivity that is optimized for thin-film based solar cell
applications. Depending on the application, the metal electrode
layer 102 may be deposited in various ways. For example, the metal
electrode layer 102 comprises primarily a film of molybdenum that
is deposited by sputtering. For example, the thickness may range
from 200 to 700 nm. A sputtering apparatus, such as a DC magnetron
sputtering apparatus, can be used to deposit a thin film of
materials upon a substrate. Such apparatus is well known and
commercially available. But it is to be understood that other types
of equipments and/or processes, such as evaporation in vacuum based
environment may be used as well. As an example, the sputtering
deposition process is described below.
[0023] Sputter deposition is a physical vapor deposition (PVD)
method of depositing thin films by sputtering, or ejecting,
material from a "target", or source, which then deposits onto a
substrate, such as a silicon wafer or glass. Sputtered atoms
ejected from the target have a wide energy distribution, typically
up to 10's of eV's (100000 K). The entire range from high-energy
ballistic impact to low-energy thermalized motion is accessible by
changing the background gas pressure. The sputtering gas is often
an inert gas such as argon. For efficient momentum transfer, the
atomic weight of the sputtering gas should be close to the atomic
weight of the target, so for sputtering light elements neon is
preferable, while for heavy elements krypton or xenon are used.
Reactive gases can also be used to sputter compounds. The compound
can be formed on the target surface, in-flight or on the substrate
depending on the process parameters. The availability of many
parameters that control sputter deposition make it a complex
process, but also allow experts a large degree of control over the
growth and microstructure of the film.
[0024] FIG. 2 is a simplified diagram of a composite structure
including copper and indium material according to an embodiment of
the present invention. This diagram is merely an example, which
should not limit the scope of the claims herein. In this
embodiment, structure 200 is includes a glass substrate 208,
preferably soda lime glass, which is about 1 to 3 millimeters
thick. For example, the glass substrate 208 serves as an supporting
layer. The metal layer 206 is deposited upon substrate 208. For
example, the metal layer 206 serves as a metal electrode layer to
provide electrical contact. For example, the layer 206 comprises
primarily a film of molybdenum which has been deposited by
sputtering to a thickness of from 200 to 700 nm. In a specific
embodiment, an initial film of chromium is first deposited upon
glass 208. For example, the chromium layer is provided to insure
good adhesion of the overall structure to the substrate 208. Other
types of material may also be used in a barrier layer, such as
silicon dioxide, silicon nitride, et al. Layers 204 and 202 include
primarily a copper layer and an indium layer deposited upon metal
layer 206 by a sputtering process. As shown in FIG. 2, the indium
layer overlays the copper layer. But it is to be understood that
other arrangements are possible. In another embodiment, the copper
layer overlays the indium layer. As an example, a sputtering
apparatus, such as a DC magnetron sputtering apparatus, is used to
deposit the thin film (e.g., layer 202, 204, and/or 206) of
materials upon a substrate. It is to be appreciated that various
types of sputtering apparatus may be used. Such apparatus is well
known and commercially available. Other material can also be used.
It is to be appreciated that techniques described throughout the
present application are flexible and that other types of equipments
and/or processes, such as evaporation in vacuum based environment
may be used as well for depositing copper and indium material. In
certain embodiments, gallium material (not shown in FIG. 2) may be
formed deposited in addition to the copper and indium material.
According to an embodiment, the ratio between the copper and indium
material is less than 1 (e.g., 0.92.about.0.96); that is, less than
one part of copper per one part of indium material.
[0025] As an example, the structure 200 is formed by processing the
structure 100. For example, the Cu and In are deposited onto the
structure 100 to form the structure 200. As described, sputtering
process is used for forming the copper and/or indium layer. In the
embodiment illustrated in FIG. 2, the Cu film and the In film are
shown as two separate layers. In another embodiment, a Cu/In
composite or Cu/In alloy is formed during the sputtering process,
as shown in FIG. 2A. It is to be appreciated that techniques
described throughout the present application are flexible and that
other types of equipments and/or processes, such as evaporation in
vacuum based environment may be used as well for depositing copper
and indium material. In certain embodiments, gallium material (not
shown in FIG. 2) may be formed deposited in addition to the copper
and indium material.
[0026] FIG. 2A is a simplified diagram of a composite structure 210
including a copper and indium composite film according to another
embodiment of the present invention. This diagram is merely an
example, which should not limit the scope of the claims herein. As
shown, the structure 210 includes a transparent substrate 216. In
an embodiment, substrate 216 can be a glass substrate, for example,
a soda lime glass. A back contact comprises a metal electrode layer
214 is deposited upon substrate 216. For example, the layer 214
comprises primarily a film of molybdenum material is deposited by
sputtering. In a specific embodiment, an initial film of chromium
is deposited upon glass 216 before depositing the chromium material
to provide for good adhesion of the overall structure to the
substrate 210. The layer 212 comprises primarily a copper indium
alloy or copper indium composite material. For example, the mixing
or alloying of copper indium results in an improved homogeneity or
advantageous morphology of the composite copper and indium film.
This improved structure is carried over into the desired CIS film
after the selenization step. According to an embodiment, an copper
indium alloy material is formed from separate layers of copper and
indium material, which diffuse into each. For example, the process
of forming of copper indium alloy material is facilitate by
providing subjecting the structure to a high temperature.
[0027] FIG. 3 is a simplified diagram of a furnace according to an
embodiment of the present invention. This diagram is merely an
example, which should not limit the scope of the claims herein. As
shown, a furnace 300 includes a process chamber 302 and a chamber
end cap 304. The inner surface and spatial region of the process
chamber 302 is represented as an inner process region 320. In an
embodiment, process chamber 302 can comprise a quartz tube. An end
cap region 322 represents the inner surface of the end cap 304 and
nearby surface of the tube that is partially exposed to the inner
process region 320. In an embodiment, end cap 304 can be made of a
metal material. In certain embodiments, the end cap 304 and the
process chamber 302 are characterized by different surface
reactivity, heat conductivity, adhesiveness, and/or other
characteristics. For example, under a certain condition various
types of materials may deposit on the end cap 304 but not on the
inner surface of chamber 302 directly exposed to the samples or
substrates to be loaded in the chamber. In a specific embodiment,
the end cap region 302 is made of material that has lower specific
heat than the process chamber 302. As shown in FIG. 3, the furnace
300 includes a vacuum-pumping machine that comprises a
turbomolecular pump 310 and a rotary pump 312. Depending on the
application, the vacuum-pumping machine can be implemented by way
of a combination of a mechanical booster pump and a dry pump. For
example, the raw material gas and/or a diluting gas such as helium,
nitrogen, argon, or hydrogen can be introduced in process chamber
302 via a gas injection pipe 314, if demanded by the specific
applications and/or processes. The chamber 302 is evacuated by the
turbomolecular pump 310 via the rotary pump 312 that is connected
with a manifold 316 via a gate valve and a conductance valve 318.
For example, there are no special partitions in the manifold or in
the reaction furnaces. A heating element 306 is mounted outside the
reaction chamber 302.
[0028] The furnace 300 can be used for many applications. According
to an embodiment, the furnace 300 is used to apply thermal energy
to various types of substrates and to introduce various types of
gaseous species, among others. In an embodiment, one or more glass
plates or substrates are positioned vertically near the center of
chamber 302. As an example, substrates 308 can be similar to those
described in FIGS. 2 and 2A (e.g., Cu/In layers or composite Cu/In
layer overlying a metal contact layer on a substrate). These layers
placed in the process chamber in the presence of a gas containing
selenium, such as H.sub.2Se. After annealing the material for a
given period of time, the copper, indium and selenium interdiffuse
and react to form a high quality copper indium diselenide (CIS)
film.
[0029] FIG. 4 is a simplified diagram of a process for forming a
copper indium diselenide layer according to an embodiment of the
present invention. This diagram is merely an example, which should
not limit the scope of the claims herein. One of ordinary skill in
the art would recognize many other variations, modifications, and
alternatives. It is also understood that the examples and
embodiments described herein are for illustrative purposes only and
that various modifications or changes in light thereof will be
suggested to persons skilled in the art and are to be included
within the spirit and purview of this process and scope of the
appended claims.
[0030] As shown in FIG. 4, the present method can be briefly
outlined below. [0031] 1. Start; [0032] 2. Provide a plurality of
substrates having a copper and indium composite structure [0033] 3.
Introduce a gaseous species including a hydrogen species and a
selenium species and a carrier gas into the furnace; [0034] 4.
Transfer thermal energy into the furnace to increase a temperature
from a first temperature to a second temperature; [0035] 5.
Maintain the temperature at about the second temperature for a
period of time; [0036] 6. Decompose any residual selenide species
from an inner region of the process region of the furnace; [0037]
7. Decrease the temperature to a third temperature; [0038] 8.
Maintain the temperature at about the third temperature for a
period of time; [0039] 9. Deposit elemental selenium species within
a vicinity of the end cap region operable at a third temperature;
[0040] 10. Ramp down the temperature from the third temperature to
about the first temperature; [0041] 11. Remove gas; and [0042] 12.
Stop.
[0043] These steps are merely examples and should not limit the
scope of the claims herein. One of ordinary skill in the art would
recognize many other variations, modifications, and alternatives.
For example, various steps outlined above may be added, removed,
modified, rearranged, repeated, and/or overlapped, as contemplated
within the scope of the invention. As shown, the method 400 begins
at start, step 402. Here, the user of the method begins at a
process chamber, such as the one noted above, as well as others.
The process chamber can be maintained at about room temperature
before proceeding with the present method.
[0044] A plurality of substrates is transferred into the process
chamber, step 402. Each of the plurality of substrates can be
provided in a vertical orientation with respect to gravity. The
plurality of substrates can be defined by a number N, where N is
greater than 5. The plurality of substrates can comprise 5 or more
individual substrates. In another embodiment, the plurality of
substrates can comprise 40 or more individual substrates. Each
substrate can have a dimension of about 65 cm to 165 cm. Each of
the substrates is maintained in substantially a planar
configuration free from warp or damage. For example, if the
substrates were provided in an orientation other than vertical with
respect to gravity, the gravitational force could cause the
substrates to sag and warp. This occurs when the substrate material
reaches a softening temperature, compromising the structural
integrity of the substrate. Typically, glass substrates, particular
soda lime glass substrates, begin to soften at 480.degree. C.
(often referred as to strain point). In an embodiment, the
substrates are also separate from one another according to a
predetermined spacing to ensure even heating and reactions with
gaseous species that are to be introduced to the furnace.
[0045] After the substrates are positioned into the process
chamber, gaseous species, including a hydrogen species, a selenium
species, and/or a carrier gas, are introduced into the process
chamber in step 406. In an embodiment, the gaseous species includes
at least a selenide species, such as H.sub.2Se, and nitrogen. In
another embodiment, the gaseous species other types of chemically
inert gas, such as helium, argon, etc. For example, the substrates
are placed in the presence of a gas containing selenium, such as
H.sub.2Se.
[0046] The furnace is then heated up to a second temperature
ranging from about 350.degree. C. to 450.degree. C. in step 408.
The transfer of thermal energy for the purpose of heating the
process chamber can be done by heating elements, heating coils, and
the like. For example, step 408, among other things, at least
starts the formation of a copper indium diselenide film by
reactions between the gaseous species and the copper and indium
composite (or layered) structure on each of the substrates. In a
specific embodiment, separate layers of copper and indium material
are diffused into each other to corm a single layer of copper
indium alloy material. The second temperature is maintained for 10
to 60 minutes (period of time) at the heat treatment interval
between 350 and 450.degree. C., step 410. In another embodiment,
the second temperature range can be from 390 to 410.degree. C. For
example, the period of time for maintaining the temperature at step
410 is provided to allow formation of the CIS film material. As the
temperature increases, the pressure inside the furnace may increase
as well. In a specific embodiment, a pressure release valve is used
to keep the pressure within the furnace at approximately 650
torr.
[0047] In an embodiment, hydrogen selenide gas H.sub.2Se can be
partially thermal cracked into the H atoms and Se vapor during the
temperature ramping up from the first temperature to the second
temperature and at the plateau of the second temperature. The Se
vapor may be partially removed in one or more processes. For
example, a cyropump can be installed for pumping out the Se vapor
directly out of the chamber. In another example, a cooled end
surface cap can serve as a cyropump to absorb or condense the Se
vapor or clusters which are carried by a convection current from
hot in-chamber processing region to the cooled end cap region,
effectively pumping out the selenium species.
[0048] During the temperature hold (step 410), additional removal
of the residual selenide species begins, in step 412. A vacuum has
been formed in the process chamber through a vacuum pump, in step
414. In a specific embodiment, the residual selenide removal
process may continue until the process chamber is in vacuum
configuration as suggested in above paragraph. Once the vacuum is
created in the process chamber (step 414), a hydrogen sulfide
(H.sub.2S) species is introduced, in step 416. The introduced
H.sub.2S, at the second temperature plateau, will induce an
exchange reaction with the selenium species incorporated in the
copper indium composite film. For example, a following reaction can
occur,
CuInSe.sub.2+H.sub.2S.fwdarw.CuInSe.sub.xS.sub.1-x+H.sub.2Se,
removing Se partially from the film on the substrate and producing
H.sub.2Se back into the environment within the chamber. At the same
time, Se particles or dust can be continuously transported by a
convective current from the hot reaction chamber to cooler regions
including the end cap region so that they can deposit on to the end
cap surface, keeping the reaction chamber substantially free of
elemental selenium. After the gas ambience in the furnace has been
changed such that the selenide species is removed and the hydrogen
sulfide species is introduced, a second temperature ramp up process
is initiated, step 418. In a specific embodiment, the selenide
species is introduced with nitrogen, which functions as a carrier
gas. The temperature of the furnace is increased to a third
temperature ranging from about 500 to 525.degree. C. For example,
the third temperature is calibrated for reaction between the
hydrogen sulfide species and the substrates in furnace. In a
preferred embodiment, the metal end cap 304 cools much faster than
the quartz process chamber 302 due to a higher thermal conductivity
of the metal end cap 304. The metal end cap 304 can stay "cool"
(substantially under 200.degree. C.) even when the chamber tube 302
is hot. A temperature gradient is created, which creates convection
current within the inner process region 320. As a result, selenium
and/or other residues are aggregated and deposited on the end cap.
In a specific embodiment, the furnace 300 as described above have
individually controllable heating units that are used to maintain
temperature uniformity within the furnace. For example, these
heaters can also be used to create a temperature difference that
causes selenium and/or other residues to move to the end cap
region.
[0049] At step 420, temperature is maintained at the third
temperature for a period of time until the formation of the CIS
layers is completed. The maintaining of time at this interval in
the ambience of the furnace comprising the sulfur species is set up
according to the purpose of extracting out one or more selenium
species from the copper indium diselenide film. It is to be
appreciate that a predetermined amount of selenium species are
removed. In a specific embodiment, approximately 5% of the selenium
is removed from the film and is replaced by about 5% of sulfur.
According to an embodiment, a complete reaction between the
selenium material with the CIS film is desired. After the removal
of selenium species, a temperature ramp down process is initiated,
in step 422. The furnace is cooled to the first temperature of
about room temperature, and the remaining gaseous species are
removed from the furnace, in step 424. For example, the gaseous
species are removed by a vacuum pumping machine. The temperature
sequence described above can be illustrated in the temperature
profile in FIG. 5.
[0050] After the decomposition of the residual selenide species, a
temperature ramp down process is initiated, in step 420. The
furnace is cooled to the first temperature of about room
temperature, and the remaining gaseous species are removed from the
furnace. In a specific embodiment, the end cap material of the
furnace is made of a specific material that cools faster than
quartz tube of the processing chamber. As a result, an air flow is
created toward the end cap (lower temperature) at the furnace,
resulting in the residues to be deposited on the end cap. In an
embodiment, the gaseous species are removed by a vacuum pumping
machine. The temperature sequence described above can be
illustrated in the temperature profile in FIG. 5.
[0051] After step 422, a final cleaning process is performed to
remove various residues deposited on the end cap of the furnace.
Depending on the condition, the residues may be removed by simply
wiping down the end cap, scrapping, polishing, and/or other
methods. It is to be appreciated that cleaning the end cap, which
is easily removable from the furnace, is much more convenient than
to cleaning the inside of a processing chamber.
[0052] Additional steps may be performed depending on the desired
end product. For example, if a CIS or CIGS type of thin-film solar
cell is desired, additional processes are provided to provide
additional structures, such as a transparent layer of material such
as ZnO overlaying the CIS layer.
[0053] It is also understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggest
to persons skilled in the art and are to be included within the
spirit and purview of this application and scope of the appended
claims.
[0054] FIGS. 5 and 5A are simplified diagrams of a temperature
profile of the furnace according to an embodiment of the present
invention. These diagrams are merely an example, which should not
limit the scope of the claims herein. The temperature profile
further details the temperature ramping process in the
above-described method outline (FIG. 4) and specification. An
optimized temperature profile (FIGS. 5 and 5A) is provided to
illustrate a heating process according to an embodiment of the
present invention. The optimized profile regulates the process
chamber in order to prevent thermal gradients and the warping of
large substrates at high temperatures. If the temperature is ramped
up too high too quickly, warping or damage may occur due to the
softening of glass. In addition, the total amount of thermal energy
is determined in consideration of total thermal budget available to
the substrates and to maintain the uniformity and structure
integrity of the glass substrate. For example, by periodically
controlling the temperature of the heating process in steps, the
substrate stays at a level of stabilization and relaxing in which
the requisite structure integrity is maintained. As explained
above, material such as glass tends to deform at a temperature of
480 degrees Celsius or higher, and thus caution is exercised to
avoid prolong exposure of substrate at high temperatures. Referring
to FIG. 5, while the ambience of a process chamber is maintained
with a gaseous species including a selenide species and a carrier
gas, a plurality of substrates is put into the furnace. In one
embodiment, air within the process chamber is pumped out before the
gaseous species are filled into the process chamber. In an
exemplary embodiment, the carrier gas comprises nitrogen gas. For
example, the gaseous species fills up the process chamber to a
pressure of about 650 Torr. The plurality of substrates is provided
in a vertical orientation with respect to a direction of gravity,
with the plurality of substrates being defined by a number N, where
N is greater than 5. In an embodiment, the substrates include glass
substrates, such as soda lime glass. The furnace is at a first
temperature of less than 100.degree. C. The furnace is then heated
up to a second temperature ranging from about 350.degree. C. to
450.degree. C.
[0055] The second temperature is maintained for 10 to 60 minutes
(period of time) at the heat treatment interval between 350 to
450.degree. C. The size of glass substrate can be, but not limited
to, 65 cm.times.165 cm. A challenge in processing such large
substrate is the warping of the substrate at high temperatures. If
the temperature is ramped up directly to T3, warping or damage may
occur. As shown, the slope of ramping up from T2 to T3 is
calibrated to reduce and/or eliminate the risk of damaging the
substrate. In one embodiment, as shown in FIG. 5A, some of the
selenide gas is removed when temperature ramps up from T2 to T3. By
maintaining the temperature in the process chamber at T2 for a
period of time, the substrate can relax and stabilize. The
maintaining time at this interval is set up according to the
purpose of at least initiating formation of the copper indium
diselenide film from the copper and indium composite structure on
each of the substrates.
[0056] The furnace is then cooled to a third temperature ranging
from about 500.degree. C. to room temperature. During the
selenization process, residual selenide species may have
accumulated in the inner process region 320 shown previously in
FIG. 3. At high temperatures, the selenide species remains in vapor
form within the inner process region 320. As the temperature
decreases, the selenide species deposits upon cooler surfaces. In a
preferred embodiment, the metal end cap 304 with no insulation
cools much faster than the quartz process chamber 302 due to a
higher thermal conductivity. A temperature gradient is created,
which creates convection current within the inner process region
320. The convection current causes the elemental selenium to flow
towards the end cap 304 and deposit on the cooler end cap region
322. Through this method, the inner process region 320 can be
maintained substantially free from elemental selenium species by
the decomposition of residual selenide species from the inner
region of the process region.
[0057] After the ambience in the furnace has been changed such that
any residual selenide species are decomposed and deposited at the
end cap region 322, a additional steps can be introduced according
to the process of forming the CIS layer on surface of the
substrates. In another embodiment, additional temperature ramping
and maintenance steps in order to prevent the substrates from
warping or becoming damaged can be used in the forming of the CIS
film.
[0058] After the formation of the CIS layer, a temperature
ramp-down process is initiated, as the furnace is then cooled to
the first temperature of about room temperature. According to an
embodiment, the cooling process is specifically calibrated. As a
result of this process, the copper, indium, and selenium
interdiffuse and react to form a high quality copper indium
diselenide film. In one embodiment, gaseous species such as
nitrogen is used to during the cooling process.
[0059] FIG. 6A is a simplified diagram of a thin film copper indium
diselenide device according to an embodiment of the present
invention. This diagram is merely an example, which should not
limit the scope of the claims herein. As shown, structure 600 is
supported on a glass substrate 610. According to an embodiment, the
glass substrate comprises soda lime glass, which is about 1 to 3
millimeters thick. A back contact including a metal layer 608 is
deposited upon substrate 610. According to an embodiment, layer 608
comprises primarily a film of molybdenum which has been deposited
by sputtering. The first active region of the structure 600
comprises a semiconductor layer 606. In an embodiment, the
semiconductor layer includes p-type copper indium diselenide (CIS)
material that is characterized by an overall thickness from 500 to
1500 .mu.m. It is to be understood that other the semiconductor
layer may include other types of material, such as CIGS. The second
active portion of the structure 600 comprises layers 604 and 602 of
n-type semiconductor material, such as CdS or ZnO. FIG. 6A shows
the second active portion of the structure 600 comprising two CdS
layers 602 and 604 with different levels of resistivity. Another
embodiment is shown in FIG. 6B, in which the second active portion
of the structure comprises both a CdS layer and a ZnO layer.
[0060] FIG. 6B is a simplified diagram of a thin film copper indium
diselenide device according to another embodiment of the present
invention. This diagram is merely an example, which should not
limit the scope of the claims herein. As shown, structure 620 is
supported on a glass substrate 630. According to an embodiment, the
glass substrate comprises soda lime glass, which is about 1 to 3
millimeters thick. A back contact including a metal layer 628 is
deposited upon substrate 630. According to an embodiment, layer 628
comprises primarily a film of molybdenum which has been deposited
by sputtering. The first active region of the structure 620
comprises a semiconductor layer 626. In an embodiment, the
semiconductor layer includes p-type copper indium diselenide (CIS)
material. It is to be understood that other the semiconductor layer
may include other types of material, such as CIGS. The second
active portion of the structure 620 comprises layers CdS 624 and
ZnO 622 of n-type semiconductor material.
[0061] A photovoltaic cell, or solar cell, such as device 600
described above, is configured as a large-area p-n junction. When
photons in sunlight hit the photovoltaic cell, the photons may be
reflected, pass through the transparent electrode layer, or become
absorbed. The semiconductor layer absorbs the energy causing
electron-hole pairs to be created. A photon needs to have greater
energy than that of the band gap in order to excite an electron
from the valence band into the conduction band. This allows the
electrons to flow through the material to produce a current. The
complementary positive charges, or holes, flow in the direction
opposite of the electrons in a photovoltaic cell. A solar panel
having many photovoltaic cells can convert solar energy into direct
current electricity.
[0062] Semiconductors based on the copper indium diselenide (CIS)
configuration are especially attractive for thin film solar cell
application because of their high optical absorption coefficients
and versatile optical and electrical characteristics. These
characteristics can in principle be manipulated and tuned for a
specific need in a given device. Selenium allows for better
uniformity across the layer and so the number of recombination
sites in the film are reduced which benefits the quantum efficiency
and thus the conversion efficiency.
[0063] The present invention provides methods for making CIS-based
and/or CIGS-based solar cells on a large glass substrate for a
solar panel. The device structure described in FIG. 6 can be
patterned into individual solar cells on the glass substrate and
interconnected to form the solar panel. A cost-effective method for
making thin film solar cell panel.
[0064] It will be appreciated that all of the benefits of the
present invention can be achieved regardless of the order of
deposition of the copper and indium films. That is, the indium
could be deposited first or the films could be deposited as a
sandwich or stack of thinner layers.
[0065] It is also understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggest
to persons skilled in the art and are to be included within the
spirit and purview of this application and scope of the appended
claims.
[0066] FIG. 7 is a simplified diagram of a self cleaning furnace
before the decomposition of the selenide species according to an
embodiment of the present invention. This diagram is merely an
example, which should not limit the scope of the claims herein. As
shown, a furnace 700 includes a process chamber 702 and a chamber
end cap 704. The inner surface and spatial region of the process
chamber 702 is represented as an inner process region 706. In an
embodiment, process chamber 702 comprises a quartz tube. An end cap
region 708 represents the inner surface of the end cap 704 that is
exposed to the inner process region 706. As described above, the
end cap 704 and the process chamber 702 are characterized by
different surface reactivity and/or adhesiveness to various types
of gaseous species. For example, under a certain condition various
types of materials may deposit on the end cap 704 but not on the
inner surface of chamber 702. In a specific embodiment, the end cap
region 702 is made of material that has lower specific heat than
the process chamber 702. In an embodiment, end cap 704 consists at
least a metal material. During the selenization process, residual
selenide 710 may deposit in the inner process region 706. According
to an embodiment, a temperature gradient can be formed by
introducing mechanical baffle between the inner process region and
the cap region so that a convective current can carry the residue
away from the inner process region and prevent the residual
selenide 710 from depositing in the inner process region 706.
[0067] It is also understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggest
to persons skilled in the art and are to be included within the
spirit and purview of this application and scope of the appended
claims.
[0068] FIG. 8A is a simplified diagram of a self cleaning furnace
before the decomposition of the selenide species according to an
embodiment of the present invention. This diagram is merely an
example, which should not limit the scope of the claims herein. As
shown, a furnace 800 includes a process chamber 802 and a chamber
end cap 804. The inner surface and spatial region of the process
chamber 802 is represented as an inner process region 806. In an
embodiment, process chamber 802 can comprise a quartz tube. An end
cap region 808 represents the inner surface of the end cap 804 that
is exposed to the inner process region 806. In an embodiment, end
cap 804 can be made of a metal material. During the selenization
process, residual selenide 810 may have deposited in the inner
process region 806. According to an embodiment, a temperature
gradient can be formed by introducing mechanical baffle between the
inner process region 806 and the cap region 808 so that a
convective current can carry the residue particles away from the
inner process region and substantially prevent the residual
selenide 810 from depositing in the inner process region 806.
[0069] FIG. 8B is a simplified diagram of a self cleaning furnace
during the decomposition of the selenide species and deposition of
elemental selenium at the end cap according to an embodiment of the
present invention. This diagram is merely an example, which should
not limit the scope of the claims herein. As shown, a furnace 820
includes a process chamber 822 and a chamber end cap 824. The inner
surface and spatial region of the process chamber 822 is
represented as an inner process region 826. In an embodiment,
process chamber 822 can comprise a quartz tube. An end cap region
828 represents the inner surface of the end cap 824 that is exposed
to the inner process region 826. In an embodiment, end cap 824 can
be made of a metal material. During the decomposition of the
selenide species, the residual species 830 including elemental
selenium becomes vaporized and flows, (carried by a convective
current induced by thermal gradient), from the inner process region
826 towards the end cap region 824.
[0070] FIG. 8C is a simplified diagram of a self cleaning furnace
after the decomposition of the selenide species and deposition of
elemental selenium at the end cap according to an embodiment of the
present invention. This diagram is merely an example, which should
not limit the scope of the claims herein. As shown, a furnace 840
includes a process chamber 842 and a chamber end cap 844. The inner
surface and spatial region of the process chamber 842 is
represented as an inner process region 846. In an embodiment,
process chamber 842 can comprise a quartz tube. An end cap region
848 represents the inner surface of the end cap 844 that is exposed
to the inner process region 846. In an embodiment, end cap 844 can
be made of a metal material. After the decomposition and deposition
of the selenide species, the residual selenide 850 is contained in
the end cap region 848. For example, the deposition or condensation
of the residue selenide 810 is caused at least in part by a
convection current due to temperature gradient between the end cap
and the process chamber. The temperature gradient can be greatly
enhanced by adding a baffle structure which serves as a controlled
permeable barrier. After the forming of the CIS layer is complete
and the substrates are removed from the process chamber 842, the
residual selenide can be mechanically cleaned with a cloth, for
example linseed cloth, or similar material. It is to be appreciated
that the cleaning process is made convenient by aggregating
selenide and/or other residues onto the end cap structure, on which
the residues can be simply wipe off. In various conventional
techniques, cleaning residues from the furnace typically requires
cleaning the interior of the processing chamber.
[0071] FIGS. 8A, 8B, and 8C are simplified diagrams of the steps
for the decomposition of the residual selenide and the path of the
elemental selenium as it deposits at the end cap region of the
process chamber.
[0072] It is also understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggest
to persons skilled in the art and are to be included within the
spirit and purview of this application and scope of the appended
claims.
[0073] FIG. 9 is a simplified diagram of a self cleaning furnace
after the decomposition of the selenide species and deposition of
elemental selenium at the end cap according to an embodiment of the
present invention. This diagram is merely an example, which should
not limit the scope of the claims herein. As shown, a furnace 900
includes a process chamber 902 and a chamber end cap 904. The inner
surface and spatial region of the process chamber 902 is
represented as an inner process region 906. In an embodiment,
process chamber 902 can comprise a quartz tube. An end cap region
908 represents the inner surface of the end cap 904 that is exposed
to the inner process region 906. In an embodiment, end cap 904 can
be made of a metal material. After the decomposition and deposition
of the selenide species, the residual selenide 910 is contained in
the end cap region 908. After the forming of the CIS layer is
complete and the substrates are removed from the process chamber
902, the residual selenide can be mechanically cleaned with a
cloth, for example linseed cloth, or similar material.
[0074] It is also understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggest
to persons skilled in the art and are to be included within the
spirit and purview of this application and scope of the appended
claims. Although the above has been generally described in terms of
a specific structure for CIS and/or CIGS thin film cells, other
specific CIS and/or CIGS configurations can also be used, such as
those noted in issued U.S. Pat. No. 4,611,091 and U.S. Pat. No.
4,612,411, which are hereby incorporated by reference herein,
without departing from the invention described by the claims
herein.
* * * * *